494* PROFESSOR FORBES ON THE DIMINUTION OF TEMPERATURE WITH 



paper, we have B = 22.4 B' = 21°.3 



dx, 21.3 



dC 1.1 



= 19.4. 



In this case, then, for every day that the epoch of the annual curve at Bo- 

 nally is retarded, the epoch of maximum difference, or quickest decrement of tem- 

 perature, is accelerated by ] 9.3 days. Now the data from which the two lower 

 curves of Plate XX. are drawn, indicate a maximum temperature at Colinton on 

 the 22d July, and a shift of the Bonally curve backwards of 4.6 days nearly. 

 The con-esponding anticipation of the quickest-decrement-epoch would therefore 

 be 4.6 X 19.4 = 89 days, which corresponds with the fact, that the greatest or- 

 dinate of the differential curve (the highest curve in the plate) evidently occurs 

 about the beginning of May. 



I have examined several other curves by means of projection in a similar 

 manner, and without pretending to any thing like a universal agreement of the 

 fonnula with the partial and dispersed observations of the kind which we pos- 

 sess, I will content myself with stating the result of a rigorous comparison with 

 the observations of Kaemtz and others already quoted, with reference to the 

 diurnal curve of decrement of temperature in the atmosphere. 



I have had computed from the empirical formulae containing four terms 

 (PoGGENDORFF, xxvii. p. 349), by which Mr Kaemtz represents the observations, 

 the epochs of maximum and minimum temperature at the Rigi and at Zurich, and 

 the epochs of quickest and slowest decrement of temperature in the course of the 

 day. 



The hours and fractions stated below are reckoned from noon. 



Now, by Eq. (7), 



dx, 



.3.1 

 2.7 



l.l.i. 



The anticipation of afternoon epoch at the Rigi compared to Zurich is P.57, 

 \\iiicli, multiplied by 1.15, gives 1''.80 for the retardation of the epoch oi quickest 

 decrement, whilst it appears to have been 2''.38. The morning epoch at the Rigi 

 anticipates by 0''.52, and the retardation of quickest decrement is 0''.29. 



From observations by the same industrious meteorologist for twenty-five 



